1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 STAT 435; 43 USC 2457).
2. Field of the Invention
The present invention generally relates to a lightweight reflector assembly and method of making the same and more particularly to an economical temperature independent reflector assembly capable of directionally diverting incident energy, such as solar energy.
3. Description of the Prior Art
Numerous examples of reflective structural elements for a variety of purposes are known in the prior art. The reflective elements have been utilized to direct various forms of energy, such as solar energy and visible light. Some forms of energy utilization have required relatively large reflectors, such as used in microwave, radio and radar antennas.
Recently, the energy crisis has required consideration of alternate sources of power, such as solar energy. In this regard, there have been suggestions to utilize relatively large reflective structures to concentrate solar energy for practical utilization. Reference is made to U.S. Pat. No. 3,884,217 issued on May 20, 1975 simply to disclose one form of an apparatus for reflecting solar energy. Generally, solar collectors provide an efficient method of making heat available for external system usage with a reasonable low heat loss for an appropriately designed reflective concentrator. Various configurations of solar concentrators have been attempted including troughs, parabolas of revolution, multiple reflectors, lenses, Fresnel reflectors, etc. One suggested solar power plant would utilize an array of concentrators (parabolas of revolution) with cavity receivers at their focus. Solar energy from the collector is piped to heat a fusion salt storage system by a pumped liquid metal heat transfer system. The pumped fluid temperature will be high enough to allow operation of a modern steam power plant using 1000.degree.-1100.degree. F steam.
To seriously consider thermionic power generation by solar energy, there must be sufficient concentrated solar radiation to provide a temperature range of 1100.degree. C. There have been various suggestions to reflect solar energy radiation from a relative large number of individually steered mirrors or heliostats to a common target receiver. This approach to solar energy collection utilizes the transmission path of reflected light to bring relatively large quantities of energy to a central location. Large flat glass reflectors of approximately six meter by six meter size have been suggested to be connected to a large steel honeycomb core. The glass face sheets are permanently bonded to the steel honeycomb core and have been suggested to have a one-half inch thickness. An alternative reflector cover has been suggested as a second surface silvered, plastic film bonded to or inserted over a 1/4 inch glass substrate.
As can be readily appreciated, solar concentrators of these types are relatively heavy and expensive. Advantageously, a solar energy collector that has a true parabolic shape pointed directly at the sun is more efficient in terms of energy collected per unit area of reflector than the flat and concentrating heliostat. The true parabolic reflective shape will provide a minimum solar image size, and therefore, a maximum concentration ratio. In addition, a parabolic collector will be more accurate in tracking the sun's relative travel.
As can be readily appreciated, the required output temperature of a solar collector to be used with a low temperature thermionic converter is approximately 1400.degree. C, and would require a relatively large structure. Since a parabolic concentrating solar collector for thermionics will optimumly have a size of approximately a 36 foot diameter with an area of approximately 1,000 square feet, and further, will be movable to accurately track the sun, a lightweight reflective structure is necessary.
The key to the implementation of solar energy as a viable source of energy depends on the design -- cost -- performance relationships as compared to conventional sources of energy. A rough estimate of the total cost for paraboloidal collectors at $50.00 per square meter of projected surface was made in 1972. The cost today has obviously, increased. It has been recognized that an economical low cost solar power plant will require the development of a solar concentrating collector which can be produced in mass quantity at a relatively low cost, yet, incorporates and maintains the necessary surface quality and rigidity to assure collection of solar energy with high efficiency. It has been calculated that the concentrator area required for a 1000 MWe power plant is over 1 .times. 10.sup.8 ft.sup.2. Obviously, a low cost solar concentrator is the most important element in the power generation.
The solar power plant efficiency is another important factor and again, depends primarily on the efficiency of concentrating the atmospherically diluted rays of the sun by the concentrating surface into a focal point at the receptor/cavity aperture for conversion into useable heat. A practical concentration ratio of approximately 1000:1 with a surface reflectivity of 0.95 percent has been suggested. This ratio would allow energy collection at 1300.degree. F with 74 percent efficiency. The 1300.degree. F will permit the generation of 1000.degree. F steam capable of driving conventional turbogenerators. The concentration ratio of 1000:1 would require a total slope error, that is, reflective surface slope and sun tracking error, of about 0.32.degree..
As any artisan in the field of optics realizes, the formation of a highly accurate concave reflective surface, or mirror, is not a problem with relatively small sizes. For example, glass blanks can be directly ground up to about 12 inches in diameter. However, this grinding technique is not applicable to large concave mirrors. A considerable amount of technical literature is available on the forming problems that exist in the field of telescope mirrors of giant size. It is not only difficult to fabricate a flawless glass blank of that diameter, but even assuming the fabrication, the frictional heat in grinding the glass blank leads to grinding dimensional inaccuracy, as well as, crazing of the surface. An example of a prior art method of forming a large collimating mirror can be found in the NASA Technical Report 32-1214 "Fabrication of the 23 ft. Collimating Mirror for the JPL 25 ft. Space Simulator" (1967). An example of a large nickel mirror obtained by electroforming the metal on a master produced by the spincasting of epoxy plastic is disclosed in the NASA Technical Memorandum 32- 206 "Solar Performance Evaluation Test Program of the 9.5 ft. Diam. Electroformed Nickel Concentrator S/N 1 at Table Mountain, California." (1967)
An additional problem exists, in that the total weight of the final mirror makes it essential to include complex supporting structures to permit the desired angular movement. Attempts have been made to utilize very thin flexible glass sheets and gently form the bonded sheets to a concave metal backing surface. The bonding, however, of thin flexible glass sheets to the concave backing substrates created dimensional inaccuracies with only relatively small temperature changes. The thermal expansions that were created produce major internal stresses, cracking and crazing to a destructive degree. As can be readily appreciated, the optimum environment for a solar concentrator would be a desert climate which is subjected to an abnormally high temperature cycle between diurnal and nocturnal periods.
U.S. Pat. No. 3,868,823 recognizes the problems of manufacturing a reflective surface, and particularly, the cost and weight problems of mirrors required to reflect the sun's rays over a sufficiently large area. This patent seeks to avoid the problem by hot pressing metal foil into an asphalt surface to form a stationary reflector and then moving a target pipe to be heated by the sun's rays.
Another fabrication procedure has suggested the grinding of a large epoxy fiberglass sandwich by a master mold with a subsequent bonding of glass, plastic or metal foil to the substrate.
To date, the prior art has not provided a lightweight, low cost thermally stable reflector assembly that can provide optimum optical reflective characteristics in a dimensionally large reflector adequate for utilization as a solar energy concentrator or a large scale antenna.